So-called Organic Light-Emitting Diodes (OLEDs) have been the focus of substantial recent attention. OLEDs are widely used in a new generation of low-power, flat-panel (and flexible) displays. Among the advantages offered by OLEDs is the possibility for preparation of very flexible displays in novel formats such as conformable panels or coatings for textiles and also the fact that backlighting is not required, so reducing energy consumption.
Whilst much of the effort has been directed towards purely organic systems (particularly in relation to light-emitting polymers), more recently metal-organic systems employing, in particular, third-row transition elements have attracted interest for use in OLED displays.i The reason for this lies in the short lifetime of the triplet excited states which are produced when charge is injected into the device. Triplet states of organic materials which are produced in OLED devices are typically long-lived as emission is a spin-forbidden process and so emission is from only one (singlet) of four (singlet plus three triplet) excited states produced. However, the presence of a heavy transition element facilitates efficient spin-orbit coupling, shortening the lifetime of the triplet states and allowing emission from all four excited states produced. In functional terms, this means that OLEDS fabricated from metal complexes could in principle emit up to four times as much light as conventional OLEDs.
Known metal-organic luminophores that emit from triplet states are based on, for example:                octahedral complexes of iridium(III) containing two N—C chelates;        square-planar complexes of platinum(II) bound to a single N—C—N chelate; and        square-planar complexes of platinum(II) bound to an N—C chelate.        
In general, existing OLED materials are vacuum deposited or spin coated as layers into devices and, as such, are present in an amorphous state, i.e. having no long range structural order. By contrast, liquid crystals do have long range structural order, when arranged in mesophases. An attractive and central feature of liquid crystal mesophases is that the organisation of the molecules therein results in attendant anisotropy of the physical properties.
Although OLED displays based on emissive metal complexes are likely to be energy efficient compared to existing technology, incorporating long range order and anisotropic properties into thin films of materials used to make metal-organic OLED displays could make a significant further increase in their energy efficiency.
Therefore, the combination of liquid crystal properties and light-emitting properties in a single compound/complex would be very desirable. For example, emission from aligned layers of calamitic (rod-like) liquid crystals leads to polarised emission. Emission of polarized light from a display reduces scattering and makes the display appear brighter. Alternatively, materials capable of forming columnar phases may lead to greatly enhanced charge carrier mobilitiesii compared to amorphous materials. This would reduce the amount of electricity needed to power a display.
These properties can be used to improve the quality of the display and to reduce the power needed for the display to work, thus extending the battery life of a portable device such as a mobile phone or a laptop computer. Other advantages of incorporating liquid crystalline properties into a metal-organic OLED display may be envisaged.
We have now surprisingly found a group of novel organo-platinum complexes which possess the property of being phosphorescent and, in some cases, are liquid crystals in which the luminescence properties in the mesophase can be controlled to be quite different from those of the amorphous material.